I think squash is way more fun. if you're in a major east coast city it's probably easier to get a court and find people to play with compared to tennis (don't need to cancel due to weather; just book tour court and show up, any decent club should have players of a variety of skill levels plus round robins, ladders, etc).

Do squash. You can play before work or after and don't need to worry about people finishing before you, like you would if you're relying on public courts to play tennis. Don't need to worry about weather. I think squash is way more intense, strategic, and has a lower initial learning curve.